The Ajax technique uses a combination of:
XHTML (or HTML), CSS, for marking up and styling information.
The XMLHttpRequest object to exchange data asynchronously with the web server. In some Ajax frameworks and in certain situations, an IFrame object is used instead of the XMLHttpRequest object to exchange data with the web server.
XML is sometimes used as the format for transferring data between the server and client, although any format will work, including preformatted HTML, plain text, JSON and even EBML.
Like DHTML, LAMP, or SPA, Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together.
Possibly the newest programming language at the moment, I predict it to be the next big thing in the web development world.
It will change the web by adding more features for users, although I haven't used it yet.